Disentangling Bulk and Surface Contributions to Charge and Discharge Fading in High‐Voltage LiCoO ₂

ABSTRACT

Increasing the charge cutoff voltage is one of the most effective strategies for improving the energy density of lithium‐ion batteries, but it also accelerates the degradation of cathode materials and leads to rapid capacity fading. For LiCoO ₂ (LCO), operation above 4.6 V is highly desirable for maximizing energy density; however, the respective roles of bulk and surface degradation in capacity fading remain unclear. Here, we compare bare and LiTaO ₃ ‐coated LCO under 4.6 V operation to decouple the bulk and surface contributions to charge and discharge fading, defined as capacity loss during charging and discharging, respectively. By tracking the accessible state‐of‐charge window through crystallographic parameters, we show that charge fading dominates during the early stage of cycling, whereas both fading modes progressively develop upon extended cycling. Through synchrotron‐based X‐ray and electrochemical analyses, we find that early‐stage charge fading is closely associated with kinetic hindrance induced by repeated transitions to the H1‐3 phase. We further find that surface reconstruction intensifies both fading modes by hindering kinetics and structural evolution, whereas the LiTaO ₃ coating suppresses these effects. This study provides a mechanistic framework elucidating how bulk and surface degradation differently contribute to capacity fading and offers guidance for designing more durable high‐energy cathodes.